Methods for controlling a fuel metering in the multiple injection operating mode

ABSTRACT

The invention relates to two methods for controlling a fuel metering of a direct-injection internal combustion engine ( 10 ) in a multiple injection operating mode according to which at least two fuel injections into a cylinder ( 12 ) are carried out during a compression cycle of the cylinder ( 12 ) of the internal combustion engine ( 10 ) by means of at least one injection valve ( 22 ). The invention provides that a pressure (rail pressure, p R ), under which the fuel is stored in front of the injection valve ( 22 ), is preset in such a manner and/or injected fuel portions of the individual injections of a combustion cycle and/or a total fuel quantity, which is injected during a multiple injection, are varied in such a manner as to prevent a forthcoming valve opening time (Δt) of the injection valve ( 22 ) from falling below a preset valve opening time (Δt K ) during at least one injection of a combustion cycle. The inventive measures ensure, with simple means, a precise fuel metering in the multiple injection operating mode and thus enable an application of the multiple injection operating mode over wide operating ranges, particularly also in the low load operating mode and idle operating mode.

[0001] The invention relates to a method for controlling fuel meteringof a direct-injection internal combustion engine in a multiple injectionoperating mode with the features of the preambles of the independentclaims 1 in 5.

[0002] Various methods are known for increasing an exhaust gastemperature of internal combustion engines after a cold start so as toaccelerate warm-up of a catalytic converter connected downstream andrender the catalytic converter ready for use.

[0003] It is known to retard an ignition angle, i.e., the time when anair-fuel mixture in a cylinder is ignited, relative to an ignition anglethat provides the highest efficiency during the warm-up phase.Retardation of the ignition angle reduces the efficiency of thecombustion while simultaneously increasing an exhaust gas temperature.The hotter exhaust gas causes the catalytic converters to heat upfaster. The method of retarding ignition reaches its limits at ignitionangles where the internal combustion engine begins to run unacceptablyrough and reliable ignition can no longer be guaranteed.

[0004] Another method for increasing the exhaust gas temperatureincludes so-called multiple injection which has recently been describedfor direct-injection, spark-ignition internal combustion engines, wherethe fuel is injected directly through injection valves into a combustionchamber of a cylinder (WO 00/08328, EP 0 982 489 A2, WO 00/57045). Inthis case, a total fuel quantity to be supplied during an operatingcycle of a cylinder is divided into two parts and supplied in twoinjection processes to the combustion chamber of the cylinder. A firstearly injection (homogeneous injection) takes place during an intakestroke of the cylinder so that the injected fuel quantity is at thefollowing ignition time at least substantially homogeneously distributedin the combustion chamber. On the other hand, a second late injection(stratified injection) is carried out during a following compressionstroke, in particular during the second half of the compression stroke,resulting in a so-called stratified charge where the injected fuel cloudis essentially concentrated in the region surrounding a spark plug ofthe cylinder. Accordingly, multiple injection operating mode of theinternal combustion engine involves a mixed operation of stratifiedcharging and homogeneous charging. The particular ignitioncharacteristic of the multiple injection operating mode results in anincreased exhaust gas temperature compared to a completely homogeneousoperation. In addition to increasing the exhaust gas temperature,multiple injection advantageously also reduces raw emission of nitricoxides NO_(x) and unburned hydrocarbons HC, thereby reducing pollutantbreakthrough during the warm-up phase.

[0005] However, precise metering of the fuel during the multipleinjection can present a problem. By dividing the total fuel quantity tobe supplied during an operating cycle over at least two injections, thevalve open times of an injection valve operated under high-pressure canbe extremely short. If the valve open times fall below a critical value,then the injection valve operates in a so-called ballistic region whichis characterized by an increasing variance of a fuel throughput. Theproblem associated with imprecise fuel metering can be aggravated underlow engine loads, particularly in idle, which results in particularlyshort valve open times due to the small required fuel quantity.Disadvantageously, no concept for feedback control of the air-fuel ratioproduced by two or more injections has been developed to date.

[0006] It is therefore an object of the present invention to propose amethod which ensures precise fuel metering in a multiple injectionoperating mode. The method should also be integratable in an existingengine control concept without adding significant process-relatedcomplexity.

[0007] The object is solved by two methods having the characterizingfeatures of the independent claims 1 and 5, which can advantageouslyalso be employed in combination.

[0008] According to the first method of the invention, a pressure, atwhich the fuel is supplied or stored before the injection valve, ispreset so that a resulting valve open time of the injection valve in atleast one injection of an operating cycle does not fall below apredetermined valve open time. Preferably, this pressure, hereinafteralso referred to as rail pressure, is preset so that the injection valvedoes not operate in a ballistic region in any of the injection processesto be carried out during the operating cycle. The rail pressure it ishereby lowered when the presently existing or required valve open timesreach or fall below the preset critical valve open time. The reducedrail pressure results in a reduced fuel throughput of the injectionvalve and hence longer valve open times since these depend on the railpressure. As a result, operation of the injection valves in theso-called ballistic region can be avoided and the fuel metering can becontrolled with high accuracy.

[0009] According to a preferred embodiment of the method, the railpressure is set to a most 35 bar, in particular to a most 30 bar,preferably to at most 25 bar. This value is significantly lower thanconventional rail pressures of 40 to 120 bar.

[0010] In general, the rail pressure can be maintained at the reducedlevel during the entire multiple injection operating mode. However, therail pressure can advantageously also be controlled as a function of anengine load and/or an engine rotation speed. In this embodiment,characteristic curves stored in an engine controller are used todetermine the optimum rail pressure as a function of the operatingpoint. The optimum rail pressure is hereby the greatest possiblepressure at which the preset critical valve open time is not underrun.

[0011] According to another method of the invention, the fuel fractionsof the individual injections of an operating cycle and/or or a totalfuel quantity injected during a multiple injection are varied in such away that a resulting valve open time during at least one injection,preferably during all injections, of an operating cycle does not fallbelow a preset valve open time.

[0012] In particular, if the fuel fractions of the individualinjections, i.e. the different valve open times, differ, then the fuelquantity injected during a shorter injection is increased at the expenseof at least one longer injection, until the shorter valve open timecorresponds at least approximately to the preset valve open time.However, this change in the fuel fractions reaches its limit where thedecreasing duration of the longer injection time reaches the criticalpreset valve open time. If this measure alone does not provide thedesired valve open times, then the rail pressure can be reduced inaddition.

[0013] In the event that all valve open times of the injections of anoperating cycle fall below the preset valve open time and/or if in spiteof the variation of the fuel fractions of the individual injections atleast one valve open time of an injection falls below the preset valveopen time, then this state can be initially tolerated according toanother embodiment of the method. The total fuel quantity suppliedduring an operating cycle is increased only if a correction imposed by alambda controller increases to a point where a predeterminable thresholdis exceeded. This is preferably implemented by increasing all fuelquantities supplied in the individual injections proportional to theirfuel fractions. This measure can advantageously also be combined with areduction in the rail pressure. Moreover, an additional useful torqueproduced by the increased total fuel quantity can be at least partiallycompensated by measures that reduce the engine efficiency, in particularby an adjustment of an ignition angle, preferably by retarding theignition timing. Instead of increasing the supplied total fuel quantityor if the supplied total fuel quantity exceeds an acceptable amount, themultiple injection operating mode may also be blocked. In this case,other heating measures for the catalytic converter, for example a singleinjection with late ignition, can be implemented.

[0014] As already mentioned several times, the measures for reducing therail pressure can be advantageously combined with changes in the fuelfractions of the individual injections and/or of the total fuelquantity, so as to ensure valve open times greater than the preset valveopen times.

[0015] The preset value of the critical valve open time dependsessentially on an acceptable variance of a fuel throughput through theinjection valve. The preset valve open time represents hereby athreshold, above which the injection valve operates with an averagevariance of the fuel throughput of a most ±20%, in particular of a most±15%, preferably of a most ±10%. Since the average variance of the fuelthroughput as a function of the valve open time depends on theparticular design of an injection valve, actual minimum valve open timeare difficult to predict. As a guideline, conventional high-pressureinjection valves can have, for example, a valve open time of 550 μs, inparticular 600 μs, preferably 700 μs, which represents a boundary limitto the ballistic region. These values can show significant upward ordownward deviations, depending on the design characteristic of thehigh-pressure injection valve. Other known valves have an acceptablevalve open time of 300 μs, in particular 350 μs, preferably 450 μs.

[0016] In addition to the problem associated with short valve open timesand the resulting imprecise fuel metering, a concept is lacking thatallows feedback control of the fuel metering of the individualinjections of an operating cycle based on a single exhaust gas signal.According to another embodiment of the method, a single injection of afuel quantity to be injected during an operating cycle can be regulated,while the fuel quantity to be injected with the at least one additionalinjection of the operating cycle can be pilot-controlled. In an actualsituation with two injections, where a first injection occurs during anintake stroke of a cylinder and a second injection occurs during acompression stroke, either the early injection can be pilot-controlledand the late injection can be regulated, or alternatively the earlyinjection can be regulated and the late injection can bepilot-controlled. The pilot control is implemented in a known mannerbased on stored characteristic curves which correlate an engine loaddemand, in particular a throttle position in an air intake duct and/or agas pedal position signal, with a required fuel quantity. Apilot-control value determined in this way for the pilot-controlledinjection is then maintained, while the corresponding other injection iscontrolled in a known manner based on a concentration of at least oneexhaust gas component measured in the exhaust gas, in particular oxygen.Alternatively, all injections of an operating cycle can be regulatedproportional to their fuel fractions to be injected. In addition, allinjections of an operating cycle can be pilot-controlled until a gassensor, in particular a lambda sensor, that measures the concentrationof the exhaust gas component, has reached its operation-ready state. Thetime delay for reaching the operation-ready state is essentiallydetermined by a minimum temperature of the gas sensor and the length ofthe exhaust gas duct between the cylinder and the gas sensor.

[0017] Additional advantageous embodiments are recited in the dependentclaims.

[0018] Embodiments of the invention will be described in more detailhereinafter with reference to the appended drawings. It is shown in:

[0019]FIG. 1 schematically the configuration of a cylinder of aninternal combustion engine with associated control elements, and

[0020]FIG. 2 a curve with an average variance of the fuel throughput asa function of the open time of an injection valve.

[0021]FIG. 1 shows an exemplary single cylinder 12 of a spark-ignitionfour-stroke internal combustion engine 10 capable of running in a leanmode. A piston 16 is axially movable in a cylinder housing 14 of thecylinder 12. A spark plug 20 with an ignition coil is located in acentral upper position of a cylinder head 18 of the cylinder housing 14;located at a lateral position is a high-pressure injection valve 22 forenabling direct fuel injection into a combustion chamber 24 of thecylinder 12. Fuel is supplied to the injection valve 22 via a fuel line26. The fuel is pumped out of a fuel tank (not shown) by a fuel pump(not shown) and the fuel pressure is reduced. A high-pressure pump 28generates a fuel pressure which in a typical vehicle operating mode isbetween 40 and 120 bar. The fuel pressure is preset depending on anoperating point of the internal combustion engine 10. The high-pressurepump 28 in cooperation with a pressure control valve (not shown)smoothes pressure variations in a fuel rail 30 disposed before theinjection valve 22. The pressure at which the fuel is supplied to theinjection valve 22 after the rail 30 is hereinafter referred to as railpressure PR. The rail pressure PR is measured with a pressure sensor 32disposed in the rail 30. An air intake channel 34 which supplies freshair terminates in the cylinder head 18 of the cylinder 12. The positionof a throttle 36 disposed in the air intake channel 34 controls an airmass flow. In addition, an outlet tube 38 terminates in the combustionchamber 24, moving exhaust gas from the combustion chamber 24 as well asexhaust gases from the other cylinders to an exhaust gas channel 40. Alambda sensor 42 arranged in the outlet tube 38 or in the exhaust gaschannel 40 measures an oxygen concentration in the exhaust gas forregulating the air-fuel ratio supplied to the internal combustion engine10 and the cylinders 12, respectively. A catalytic converter 44 arrangedin the exhaust gas channel 40 converts pollutants contained in theexhaust gas. An operating state of the internal combustion engine 10 iscontrolled by an engine controller 46, taking into account variousavailable operating data.

[0022] The catalytic converter 44 has typically not yet reached anoperating temperature required for adequate conversion of pollutants,particularly after an engine cold start. This is recognized by theengine controller 46, for example by a temperature sensor signalmeasured before, after or inside the catalytic converter 44. To increasean exhaust gas temperature and to accelerate warm-up of the catalyticconverter 44, the motor controller 46 switches the internal combustionengine 10 to a multiple injection operating mode. During an intakestroke of the cylinder 12, a first early fuel injection occurs throughthe injection valve 22, while a second late fuel injection takes placein a following compression stroke, in particular during the second halfof the compression stroke. The corresponding injection angles α_(E) andvalve open times Δt of the injection valve 22 as well as an ignitionangle α_(z) of the spark plug 20 are preset by the engine controller 46.The fuel supplied during the early injection is present in thecombustion chamber 24 at the time of ignition α_(z) in the form of anessentially homogeneous mixture. Conversely, the fuel supplied duringthe late injection forms a stratified charge cloud which is concentratedat the ignition time α_(z) in the region of the spark plug 20. Theformation of the so-called stratified charge is aided by providing thepiston head of piston 16 with a special trough-shaped contour.Distributing the total fuel quantity supplied during an operating cycleof the cylinder 12 over two injection processes requires very shortvalve open times Δt of the injection valve 22, which can cause problemsresulting in imprecise fuel metering.

[0023] A average percentage variance of the fuel throughput σ_(KS) as afunction of the valve open time Δt is shown more clearly in FIG. 2.Above a valve open time of approximately 700 μs, the fuel throughput hasan small, approximately constant variance of approximately 5%. BelowΔt_(b), the injection valve 22 operates in the so-called ballisticregion, which is characterized by a rapidly increasing variance of thefuel throughput quantity σ_(KS) with decreasing valve open times. Thisstochastic variance of the fuel metering between the individualoperating cycles of the cylinder 12 causes the lambda control tocontinuously and frequently intervene, accompanied by a generallyimprecise adjustment of the air-fuel ratio. With a typical rail pressurep_(R) of 40 to 120 bar when the internal combustion engine 10 is idling,the valve open times of the two injections in multiple injection modeare typically in the range of 350 to 500 μs and hence in the inaccurateballistic region. According to the invention, the rail pressure p_(R)and/or the total supplied fuel quantity and/or the fractions of the fuelquantities to be supplied during the injections as part of the totalfuel quantity are to be preset and/or to be varied such that they do notfall below a preset critical valve open time Δt_(K). According to theexemplary curve depicted in FIG. 2, a critical valve open time Δt_(K) ofapproximately 500 μs would have to be set for a maximum acceptablevariance of the fuel metering of approximately ±10%. Preferably,however, a more conservative value is preset that is close to or equalto the ballistic limit Δt_(b). It should be mentioned again that thedepicted curve as well as the described values are only exemplary anddepend strongly on the design characteristic of the injection valve 22.

[0024] If the engine controller 46 recognizes that the valve open timeΔt of the early and/or of the late injection is less than the presetopening time Δt_(K), then the controller 46 initially increases the fuelquantity to be supplied during the shorter injection at the expense ofthe longer injection. This adjustment of the injected fractions reachesits limit where the longer injection time is reduced so far that itapproaches the ballistic region of the injection valve 22. If bothinjections are in the ballistic region, then this situation is tolerateduntil a correction initiated by the lambda controller exceeds apredetermined threshold value. Only then is the total supplied fuelquantity increased, whereby the valve open times Δt of both injectionsare increased so that preferably both injections operate with valve opentimes Δt above the predetermined limit Δt_(K). A useful engine torquegenerated by the increased total fuel quantity can be more or lesscompensated by adjusting, preferably retarding, the ignition angleα_(z). Alternatively or in addition, the rail pressure p_(R) in themultiple injection operating mode can be lowered, preferably to lessthan 25 bar. Advantageously, the reduction of the rail pressure can becontrolled as a function of an engine load expressed by a pedal positionsignal PWG and/or an engine rotation speed n. A lower rail pressurep_(R) is preset for smaller engine loads, where only a small quantity offuel is required and the valve open times Δt are therefore short, thanfor large engine loads.

[0025] In addition, one of the two injections in multiple injectionoperating mode, for example the early injection during the intakestroke, can be pilot-controlled and the other injection, for example thelate injection during a compression stroke, can be regulated. The enginecontroller 46 hereby determines a required air mass flow as a functionof the requested engine load, in particular based on a pedal positionsignal PWG, and the throttle 36 is controlled accordingly. At the sametime or depending on the throttle position, the engine controller 46computes the required total fuel quantity to be supplied and distributesthe total fuel quantity over the two fuel injections according to anoptimal parameter choice. The valve open time Δt of the injection valve22 for the pilot-controlled, early injection is preset based on thestored characteristic curves and held constant. Conversely, theregulated, late injection is controlled via the lambda signal measuredby the lambda sensor 42 in the exhaust gas. The valve open time Δt ofthe late injection is hereby adjusted depending on a deviation of themeasured lambda value from a nominal setting. Alternatively, the valveopen times Δt of both injections can also be controlled proportional totheir fraction of the total valve open time and/or the total injectedfuel quantity.

[0026] The measures according to the invention make it possible toeasily, accurately and reliably meter the fuel in a multiple injectionoperating mode. The multiple injection operating mode can then beimplemented over wide operating ranges, in particular when operatingunder low-load or idling. The method can be easily integrated in anexisting engine controller configuration without increasing itscomplexity.

[0027] List of Reference Numerals

[0028]10 internal combustion engine

[0029]12 cylinder

[0030]14 cylinder housing

[0031]16, piston

[0032]18 cylinder head

[0033]20 spark plug

[0034]22 injection valve

[0035]24 combustion chamber

[0036]26, fuel line.

[0037]28 high-pressure pump

[0038]30 fuel rail

[0039]32 pressure sensor

[0040]34 air intake channel

[0041]36 throttle

[0042]38 outlet tube

[0043]40 exhaust gas channel

[0044]42 gas sensor/lambda sensor

[0045]44 catalytic converter

[0046]46 engine controller

[0047] α_(E) injection angle

[0048] α_(Z) ignition angle

[0049] λ air value lambda

[0050] Δt valve open time

[0051] Δt_(b) limit of the valve open time towards ballistic region

[0052] Δt_(K) critical valve open time

[0053] n engine rotation speed

[0054] p_(R) rail pressure

[0055] PWG pedal position signal

[0056] σ_(KS) average variance of the fuel throughput

1. Method for controlling fuel metering of a direct-injection internalcombustion engine (10) in a multiple injection operating mode, whereinat least two fuel injections into a cylinder (12) occur through aninjection valve (22) during an operating cycle of a cylinder (12) of theinternal combustion engine (10), characterized in that a pressure (railpressure, p_(R)), at which the fuel is supplied before the injectionvalve (22), is preset so that a resulting valve open time (Δt) of theinjection valve (22) in at least one injection of an operating cycledoes not fall below a predetermined valve open time (Δt_(K)).
 2. Methodaccording to claim 1, characterized in that the rail pressure (p_(R)) isset to a most 35 bar.
 3. Method according to claim 2, characterized inthat the rail pressure (p_(R)) is set to a most 30 bar, in particular toa most 25 bar.
 4. Method according to one of the claims 1 to 3,characterized in that the rail pressure (PR) is controlled as a functionof an engine load and/or an engine rotation speed.
 5. Method forcontrolling fuel metering of a direct-injection internal combustionengine (10) in a multiple injection operating mode, wherein at least twofuel injections into a cylinder (12) occur through an injection valve(22) during an operating cycle of a cylinder (12) of the internalcombustion engine (10), characterized in that injected fuel fractions ofthe individual injections of an operating cycle and/or a total fuelquantity injected during a multiple injection are varied in such a waythat a resulting valve open time (Δt) in at least one injection of anoperating cycle does not fall below a preset valve open time (Δt_(K)).6. Method according to claim 5, characterized in that if the fuelfractions and valve open times (Δt) of the individual injections of theoperating cycle differ, then the fuel quantity injected during a shorterinjection is increased at the expense of at least one longer injectionuntil the shorter valve open time (Δt) corresponds at leastapproximately to the preset valve open time (Δt_(K)).
 7. Methodaccording to claim 5 or 6, characterized in that, if all valve opentimes (Δt) of the injections of an operating cycle fall below the presetvalve open time (Δt_(K)) and/or in spite of the variation of the fuelfractions of the individual injections at least one valve open time (Δt)of an injection falls below the preset valve open time (Δt_(K)), thenthe total fuel quantity supplied during an operating cycle is onlyincreased when a control intervention of a lambda controller exceeds apresettable threshold.
 8. Method according to one of the claims 5 to 7,characterized in that a useful torque produced by the increased totalfuel quantity is at least partially compensated by measures that reducethe engine efficiency, in particular by an adjustment of an ignitionangle.
 9. Method according to one of the claims 5 to 8, characterized inthat if all valve open times (Δt) of the injections of an operatingcycle fall below the preset valve open time (Δt_(K)) and/or if in spiteof a variation of the fuel fractions of the individual injections atleast one valve open time (Δt) of an injection falls below the presetvalve open time (Δt_(K)), then the multiple injection operating mode isblocked.
 10. Method according to one of the preceding claims,characterized in that above the preset valve open time (Δt_(K)) theinjection valve (22) operates with an average variance of a fuelthroughput (σ_(KS)) of a most ±20%, in particular of a most ±15%, inparticular of a most ±10%.
 11. Method according to one of the precedingclaims, characterized in that the preset valve open time (Δt_(K)) is 550μs, in particular 600 μs, in particular 700 μs.
 12. Method according toclaim 11, characterized in that the preset valve open time (Δt_(K)) is300 μs, in particular 350 μs, in particular 450 μs.
 13. Method accordingto one of the preceding claims, characterized in that a fuel quantity tobe injected during an operating cycle is regulated based on a deviationof an actual fuel quantity from a nominal fuel quantity and that a fuelquantity to be injected through the at least one additional injection ofthe operating cycle is pilot-controlled.
 14. Method according to one ofthe preceding claims, characterized in that all injections of anoperating cycle are regulated proportional to their fuel fractions to beinjected depending on the difference between the actual fuel quantityand the nominal fuel quantity.
 15. Method according to claim 13 or 14,characterized in that all injections of an operating cycle arepilot-controlled until a gas sensor, in particular a lambda sensor, thatmeasures the concentration of the exhaust gas component has reached itsoperation-ready state.
 16. Method according to one of the claims 13 to15, characterized in that the pilot control performed usingcharacteristic parameter fields that depend on the engine load. 17.Method according to one of the claims 13 to 16, characterized in thatthe nominal fuel quantity to be injected is regulated based on aconcentration of at least one exhaust gas component, in particularoxygen, measured in the exhaust gas.